1. Field of the invention
The invention relates to transmission systems incorporating signal transmission repeater stations for transmitting and distributing television signals for example.
2. Description of the prior art
The distribution of television signals, or "teledistribution", on co-axial transmission cables is a typical example of wide band transmission. It is known that the transmission of a single channel of television signals requires a frequency band width of several megahertz. In teledistribution by co-axial cable where several television channels are transmitted simultaneously to permit a selection of programmes by the user, the comprehensive bandwidth required for transmission is therefore large, and it is frequently between 30 and 300 MHz.
A co-axial cable has distributed losses which increase with the length of the cable. It is known to compensate for these losses by inserting repeater stations at intervals along the coaxial cable to boost the amplitude of the transmitted signals. Generally, the repeater stations are installed at regular intervals corresponding to a length of cable over which the loss is 21 dB for a 300 MHz signal. Thus, the distance between repeater stations depends upon the type of cable used. As the creation of a junction on the cable also requires an amplification, each repeater station is provided with an auxiliary output.
One difficulty is that the losses for different channels along the co-axial cable are unequal since the losses are a function of the signal frequency, and another is that the losses are variable as a function of the temperature. Moreover, it is difficult to locate the repeaters at strictly regular intervals, corresponding to an attenuation of 21 DB for a 300 MHz signal. As a result the repeater stations are more numerous and located closer to one another than is strictly necessary. A simple repeater station, while preserving the transmission signals and preventing the transmitted signals from being progressively drowned by the noise, does not compensate the inequalities of the losses which are a function of frequency, or their variation with temperature changes.
Repeater stations have incorporated a device called an equaliser or leveller, the task of which is to reduce the difference in losses for different frequencies, and to compensate for the effects of temperature variations. The basic element of such devices is a circuit whose frequency response is made in some way adjustable, and complex regulation means is required for effecting this adjustment.
These systems are not completely satisfactory; either the adjustment possibilities of the frequency response are reduced, which manifests itself in practice by a very imperfect equalisation but which can readily be achieved by automatic means, or these possibilities are very developed which requires complex adjustments requiring the obtaining of detailed data on the real value of the losses to obtain a good equalisation. Advanced intervention of technicians is required in this second instance, and only passes on the technical problems to the design and maintenance stages.
The quality required for equalisation increases with the length of the coaxial cable and of the transmission system. If it is considered that tele-distribution installations cover a substantial geographical area, and aim at technical visits as removed as possible one from the other, it is clear that the solutions set out above are far from meeting this double requirement.
The object of the present invention is to provide an improved transmission system.
The invention also has for its object a simple means intended to automatically equalise and fix the values of the losses in a coaxial transmission cable.